1. Technical Field
The invention relates to loudspeakers and more particularly to planar or ribbon type loudspeakers fitted to be used in a hanging array of loudspeakers.
2. Description of the Problem
Electro-dynamic planar loudspeakers are constructed using a thin resistive film diaphragm disposed taut over a thin, rectangular frame. Permanent magnets are mounted within the frame parallel with at least one, and sometimes both, of the major surfaces of the thin film diaphragm. In prior art designs, the bar magnets align with the axis of the direction of elongation of the frame. Typically one major surface of the thin film diaphragm carries electrically conductive traces which are etched from a layer of aluminum applied to the major surface. The conductive traces are positioned with respect to the permanent magnets so that electrical currents applied to the conductive traces result in forces being generated which move the diaphragm. The major surfaces of the diaphragm are enclosed between the magnets adjacent one major surface and, typically, a grate or sound lens adjacent the opposite major surface.
The introduction of a varying current to the electrically conductive traces causes motive force to be generated on the thin film conductors, and corresponding movement of the thin film diaphragm in the frame. The diaphragm and electrically conductive traces are of minimal mass to minimize energy expended moving the diaphragm and to optimize response times. A carefully calibrated and even tension should be applied to the diaphragm along its entire perimeter. Consistent spacing between the diaphragm and the magnets contributes to minimization of distortion in sound reproduction.
Planar type loudspeakers have a defined acoustical directivity pattern that has a natural application to vertically hung arrays. In such an array a long chain of speakers produces a sound field having broad area coverage. Such arrays are usually constructed to curve rearwardly at the bottom of the curve to produce a shallow J-shaped or fish hook profile. This arrangement orients the lowest loudspeakers in the chain to present their sound emitting surfaces directly toward the portion of the audience sitting below the array. A J-shaped curved array typically has an elongated upper portion which is substantially straight, from which sound is directed outwardly, and a lower, curved portion, from which sound is directed downwardly.
It is usually desirable that listeners anywhere in the covered area hear substantially the same sound. Achieving this result is very difficult in practice. One difficulty has been that the lower, curved portion of the J curve array has exhibited excessively large granularity. The term “granularity” is used to refer to the number and size of dead zones vertically disposed in a sound field. With conventional planar loudspeakers, the fewer the number of dead zones, the larger the dead zones tend to be. Fine granularity splits up dead zones to reduce their size below the threshold of human detection. The use of straight planar loudspeakers in the curved portions of a hanging array results in the appearance of obtrusive dead areas in the projected sound field. The lowest practical limit on the length of planar loudspeakers still results in J-curve arrays where the loudspeakers poorly fit the curve of the array. This results in substantial dead spots in the sound field. It is desirable to avoid use of short planar loudspeakers on account of their expense, power handling considerations and the poor low frequency response of very short planar devices.